10.5. Namespaces and XPath Expressions

It's
time to remember that
default namespaces do not apply to
XPath expressions, and that a
prefix needs to be defined
for each namespace used in the XPath expressions of the
xs:unique,
xs:key, and
xs:keyref declarations!

A prefix declaration is therefore required to write expressions that
reference qualified elements and attributes, even if the target
namespace is defined as the default namespace. It's
also important to notice that the xs:keyrefrefer attribute is a
QName that needs to be prefixed, and a xs:key/xs:keyref on the authors in (in our previous
example) with a namespace could become: